The invention relates to a method for operating a coal-fired power plant by using a dry-firing furnace, wherein the flue gas stream is guided through an NO.sub.x removal device, an air preheater, a dust separator, and a desulfurization device.
Flue gas streams of combustion devices must be scrubbed according to the requirements of environmental protection and the service life of the power plant of undesired components which are detrimental to the environment and/or the parts of the power plant guiding the flue gas stream.
Flue gas streams contain in addition to solid particles, which can be relatively easily filtered, detrimental admixed components which in gas streams of a sufficiently high temperature are gaseous. These detrimental admixed components in combustion devices which are using fossil fuels, are, for example, acid-forming gases such as SO.sub.2, NO.sub.x, SO.sub.3, and hydrogen halides.
For the separation of individual gases from the flue gas stream a plurality of methods operating according to different physical-chemical principles at different gas temperatures is known. In general, a cooling of the flue gas stream between the furnace and the flue gas smoke stack takes place. Due to energy considerations, for example, the efficiency of the power plant, and with regard to environmental considerations, it is in general desired that the temperature of the flue gas stream at the end of the path, i.e., at the smoke stack, be low.
This leads to the fact that at some locations of the flue gas path the dew point temperature at least of some of the acid-forming gas components is reached and that condensation of acids at the inner parts of the power plant takes place. This condensation of acids in the flue gas path of power plants, especially of sulfuric acid and hydrogen halides, results in a great stress of the power plant parts. The parts must therefore be protected with high expenditures, for example, by enameling and coating.
For example, in conventional power plants the cooling of the flue gas stream below the dew point of sulfuric acid and hydrogen halide takes place within a heat exchanger that is used for preheating the combustion air guided into the furnace. In inexpensive regenerative heat exchangers the inner parts must therefore be coated with enamel, must be cleaned frequently, and exchanged often whereby in addition to the maintenance expenditures for servicing the heat exchanger further expenses are incurred due to the required downtime of the combustion device.
Applicant has described a method for removing undesired gaseous components in German Patent Application P 41 13 793.0-43 in which at one location of the flue gas path, where the flue gas has a temperature above the dew point temperature of the undesired gaseous component, alkaline earth oxides or hydroxides are introduced. The flue gas stream is then lowered to a temperature below the dew point temperature of the undesired component whereby at least one part of the undesired component in conjunction with the introduced alkaline earth oxides and/or hydroxides are transformed into a solid material.
In experiments after start-up of modern NO.sub.x removal devices in coal-fired power plants it has been found that the limited service life of the power plant parts downstream of the furnace, especially of the air preheater, the flue gas channels, the electrostatic filter, the forced draft channels, the desulfurization device, and the gas preheating device, has substantially two causes:
a) High flue gas temperatures during full load of 390.degree. C., in conjunction with a high conversion rate of SO.sub.2 /SO.sub.3 (at 390.degree. C. approximately 3%, at 350.degree. C. at most 1.1%). PA1 b) The low flue gas temperature of less than 320.degree. C. at partial load, in conjunction with an oversalting of the catalyst that is part of the NO.sub.x removal device. PA1 supplying coal as a fuel; PA1 adding a Ca-containing substance to the coal; PA1 adjusting the amount of Ca-containing substance such that after addition a CaO contents of at least 0.3% is obtained; PA1 burning the coal with the added Ca-containing substance in the dry-firing furnace whereby a flue gas is produced; and PA1 guiding the flue gas through a NO.sub.x removal device, an air preheater, a particle separator, and a desulfurization device. PA1 1. Reaction of SO.sub.3 and CaO to CaSO.sub.4 ; PA1 2. Conversion damping within the catalyst due to the increased alkalinity (increased CaO contents).
To date a high conversion rate of up to 3% also resulted in an increase of the acid dew point of 90.degree. C. to approximately 125.degree. C. In the air preheater contamination were present that could no longer be scrubbed with the blowers. This resulted in deposits within the downstream parts of the flue gas path, for example, in the electrostatic filter, the flue gas channels, in the forced draft channels, and in the desulfurization device due to the higher dew point as well as an increased corrosion load.
It is therefore an object of the invention to improve the aforementioned method such that the service life of the power plant parts downstream of the furnace within the flue gas path can be increased without detrimental side effects and that deposits and corrosion stress can be reduced.